Malignant knee tumors are frequently located in the femur or in the tibia close to the knee joint so that, after removal of the affected bone portion, part of the shaft as well as the associated femoral or tibial joint section must be replaced by artificial prosthetic elements.
In any case, a prosthetic part must also be implanted in the corresponding bones in order to achieve articulation and coupling with the prosthetic element of the affected bone. Coupling is by means of a joint device such as a hinge or ball mechanism, which allows mobility between the femur and the tibia that is as close as possible to the natural mobility of the knee joint. The prosthesis is anchored in the femur and in the tibia in each case by means of a shaft anchoring means introduced into the bone. If later removal is intended, the shaft anchoring means can be formed so as to have a smooth surface so that bone does not grow into it. If definitive anchoring is intended, the surface can be roughened or coated so that bone tissue is able to grow into it. Alternatively, the shaft anchoring means can also be fixed by means of bone cement.
Since malignant bone tumors frequently occur during the growth phase, the loss of the growth plate on the affected side and continued growth on the opposite side result in a leg length discrepancy following surgical removal of the tumor and the implanting of a tumor prosthesis. The retarded growth affects primarily the bone affected by the tumor itself but also the corresponding bone, that is to say, if the tumor was located in the femur, for example, growth not only of the femur but also of the tibia is retarded. Even if the shaft anchoring part of the tibia is in this case smooth and polished so that the bone can in principle continue to grow, there is thus a growth retardation also in the corresponding bone, which contributes to the developing leg length discrepancy.
Possibilities for lengthening the bone affected by the tumor are known from EP 1 371 346 A1 and EP 2 468 216 A1. To that end, EP 1 371 346 A1 discloses, by means of the prosthesis, exchanging the shaft anchoring means in the remaining bones for an intramedullary lengthening nail which, following an osteotomy, slowly pulls the two bone fragments apart so that new bone tissue can form in the growing gap (callus distraction method). When the tumor is located in the proximal femur, the exchange is likewise made proximally, and when the tumor is located in the distal femur, the exchange is likewise made distally, decoupling of the prosthesis not being absolutely essential from an anatomical point of view because the point of entry can be reached axially with the knee bent for tools and the shaft anchoring parts or also an intramedullary distraction nail. When the tumor is located in the proximal tibia, the tibial anchoring means cannot be exchanged either with the knee extended or with the knee bent without decoupling the prosthetic elements, because the axial access is covered by the femoral prosthetic element.
EP 2 468 216 A1 discloses for the first time a solution for exchanging the shaft anchoring means in the femur by a minimally invasive operating technique by changing the components from the opposite end of the bone. Accordingly, if the tumor is located in the femur and there is no length discrepancy in the lower leg, a further major intervention via the prosthesis can be avoided by means of a prosthesis according to EP 2 468 216 A1, and the risk of infection can thus be reduced significantly.
Where the tibia requires lengthening, either following a primary tumor of the tibia or if the tibia is also shortened following a tumor located in the femur, and the temporary shaft anchoring means in the tibia is therefore to be exchanged for an intramedullary lengthening nail and, when growth is complete, the intramedullary lengthening nail is to be exchanged for a coated shaft anchoring means which grows permanently into the bone, the prosthesis must be decoupled each time, which requires a major surgical intervention having a considerable risk of infection.
The advantage described in EP 2 468 216 A1 thus ultimately comes to bear only if there is not a shortening of the lower leg at the same time, since the solution described in EP 2 468 216 A1 cannot be applied to the lower leg. However, for the frequent case that the shortening also affects the lower leg, as already mentioned, there is as yet no possibility for carrying out a lengthening of the type described in EP 1 371 346 A1 by a minimally invasive technique without decoupling the prosthesis. Accordingly, the advantageous possibilities of leg length equalization on the opposite side by bone growth continue to be accompanied by the considerable risk of infection due to repeated major surgical interventions in the region of the prosthesis.
Accordingly, an aim of the present disclosure is to reduce the extent of further surgical interventions and thus the risk of infection also for lengthening the tibia, while retaining the advantages of bone lengthening by the callus distraction method.